For more than a decade, CWRU has been performing scientific research on a new concept of alloy surface engineering: case hardening (generating a “hard shell”) by CSS (colossal supersaturation) with interstitial solutes. Interstitial solute refers to small atoms like carbon or nitrogen, which reside in small spaces between the regular positions of the (“substitutional”) metal atoms in the crystal lattice of the alloy. CSS refers to a state in which the solute concentration is much (orders of magnitude) higher than the equilibrium solubility limit. For CSS surface hardening, alloy parts are exposed to a gas phase that provides interstitial solute (carbon or nitrogen) atoms to diffuse into the alloy surface. It has been shown for a broad variety of structural alloys (stainless steels, nickel-base alloys, cobalt-base alloys, titanium-base alloys) that high concentrations of interstitial solute significantly enhance the mechanical properties (hardness, wear resistance, and fatigue life) and the corrosion resistance. In addition to well-established alloy performance benefits, CSS is applied as a highly conformal post process to components in their final shape—without changing their dimensions—and at low cost. Therefore, CSS can make a very important contribution for developing better, safer, and longer-lasting parts of structural alloys. The potential for technical applications is tremendous.
The principle of surface engineering by CSS is well understood. Sufficiently high concentrations of interstitial solute atoms can be dissolved if the following conditions are fulfilled: (i) The alloys contain an element with a high affinity for the interstitial solute. (ii) The processing temperature is chosen such that the interstitial solute atoms can diffuse sufficiently fast for obtaining technically useful case depths in technically feasible amounts of processing time, but low enough to immobilize the (“substitutional”) metal atoms, such that the interstitial solute atoms cannot precipitate with them as e.g. carbides or nitrides, which would be detrimental to the properties.
Surface Activation
It was found experimentally that for many alloys the native alloy surface is not transparent for inward diffusion of interstitial solute (carbon or nitrogen) at the (low) temperatures required for CSS. The reasons for this may not be perfectly understood. The current hypothesis is that effective infusion with carbon or nitrogen requires the removal of a thin barrier layer on the alloy surface. Making the surface transparent to inward-diffusing interstitial solute atoms is known as “surface activation.”
In a broad variety of alloys that were studied, condition (i) is fulfilled by a significant fraction of Cr (chromium), an element with high affinity for carbon and nitrogen. Alloys with a suitable level of Cr contain this element typically to make them corrosion resistant. The corrosion resistance follows from the ability of Cr to form a thin (≈1 nm), passivating (“sealing”) oxide layer on the surface. The oxide in this layer is typically rich in Cr. The problem that arises from this situation is that at processing temperatures that fulfill condition (ii), the passivating oxide layer may stay intact and obstruct the inward diffusion of carbon or nitrogen.
Another potential obstacle for the infusion of carbon or nitrogen into the surface of alloy parts with machined surfaces could be that the machining introduces heavy plastic deformation and contamination of the region closest to the surface and therefore leaves behind a thin layer with poor crystallinity and poor transparency for carbon or nitrogen.
Intense research over the past decade has revealed that for successful CSS case hardening, it is of greatest importance to effectively activate the surface, i.e. to make it transparent to inward diffusing carbon or nitrogen atoms. This finding confirms what was found earlier in U.S. Pat. No. 6,165,597, which discloses a procedure to activate the surface (i.e. remove oxide and/or damaged layer) with the help of HCl gas. This method of surface activation constitutes a key element for the process, for which FIG. 1 shows a temperature-time diagram. Once the surface has been activated, it stays active while infusion of interstitial solute proceeds.
Problems of Existing Technology
The problems of the existing technology, i.e. surface activation with the help of HCl gas at elevated temperature—250 to 450° C.—are numerous:
The activation by hot HCl gas is carried out in the CSS processing furnace, posing limits on process design.
The hot HCl gas causes rapid corrosion of the furnace hardware, which constitutes a significant cost factor.
The HCl gas is problematic under the aspects of safety and environmental pollution and sustainability.
In the current industrial process, surface activation requires a total of four hours of processing time.
The above factors imply increased production costs.